Now that is not something you see everyday!
— The Sniper
Quality is a property of items, which is represented in-game by the color of the item's name and (in most cases) by a title prefixed to the item's name. A dropped item can only have one quality, and an unboxed item can have 1 or 2 qualities. Extra qualities can be added through the use of strangifiers or unusualifiers.
Quality is a purely cosmetic property, and has no effect on gameplay other than some additional on-screen messages - these messages usually relay succeeded operations with an item.
Possible weapon qualities
Please see the table listed here.
Possible cosmetic item qualities
April 29, 2008 Patch
(Gold Rush Update
- [Undocumented] Added Unique quality.
March 18, 2010 Patch (First Community Contribution Update)
- [Undocumented] Added Self-Made quality.
September 30, 2010 Patch (Mann-Conomy Update)
- [Undocumented] Added Vintage quality.
- [Undocumented] Added Unusual quality.
February 3, 2011 Patch
- [Undocumented] Added Genuine quality.
June 23, 2011 Patch (Über Update)
- [Undocumented] Added Strange quality.
August 18, 2011 Patch
- [Undocumented] Added string for potential new item quality called "Completed".
September 2, 2011 Patch
- Updated the Strange quality color from gold (#CD9B1D) to orange (#CF6A32).
- [Undocumented] Updated the Rarity3 quality color from orange (#CF6A32) to brown (#70550F).
October 27, 2011 Patch (Very Scary Halloween Special)
- [Undocumented] Added Haunted quality.
June 11, 2012 Patch
- [Undocumented] Added "tobor_a" quality.
October 26, 2012 Patch (Spectral Halloween Special)
- [Undocumented] Updated Haunted quality color from #8650AC to #38F3AB.
March 19, 2013 Patch
- [Undocumented] Removed "tobor_a" quality.
November 12, 2013 Patch
- Added "Collector's" item quality with color #830000. "Collector's" items are created by completing special Chemistry Sets.
November 26, 2013 Patch
- [Undocumented] Updated Collector's quality color from #830000 to #AA0000.
September 17, 2014 Patch
May 28, 2015 [Item schema update]
- Added "restriction" and "map" quality.
July 2, 2015 Patch (Gun Mettle Update)
- [Undocumented] Added Decorated quality.
- Two unused item qualities referred to as Rarity2 (mentioned as Customized) and Rarity3 (mentioned as Well-Designed and Artisan) are specified in, the game's files. Their colors are two shades of brown; #8D834B and #70550F respectively.
- The "Completed" quality is specified in the game's files, and its color is the same as the Unusual quality: #8650AC.